A problem in decoding the modulated carriers occurs if the carriers are subject to fading or interference. The problem is particularly acute if fading or interference affects the carriers over a narrow band of frequency since, although there may a negligible effect on the majority of carriers, it causes a large effect on a few carriers. The problem may be the result of frequency selective fading caused by multipath interference or may be the result of narrow band interference caused by analog television signals.
The use of patterns of erasures to increase the rate of error correction of Viterbi decoded convolutional codes is already known. This technique, known as puncturing, uses short repetitive sequences of erasures and achieves very good performance. Simulations using random patterns of erasures for puncturing give results that are only slightly inferior to the best known repetitive sequences.
Rather than simply marking a symbol as erased or not erased, an error probability can be assigned to the symbol to represent information about the reliability of the received symbol. This technique is widely used in the so-called soft decision Viterbi decoder and in its standard from the information on the reliability of the symbol is derived solely from the quality of the received symbol, usually as a function of its Euclidean distance from a decision threshold.
In an extended form of the Viterbi decoder, soft decision information is derived from the state of the channel. One technique is to determine the signal to noise ratio for each carrier and to use this information to derive a measure of the error probability for the symbols assigned to that carrier. One method for determining this error probability is to use a null symbol, during which there is an interruption in the transmission, and to allow the receiver to measure the spectral response of the noise and interference and thereby infer the interference power and hence error probability for each carrier.
A problem with the use of null symbols is that the data rate of the transmission is degraded by the need to accommodate the null symbols.